1. Field of the Invention
The present invention relates to a nuclear quadrupole resonance (NQR) method and probe for generating RF magnetic fields in different directions towards a sample. More specifically, the present invention relates to an NQR method and probe for generating RF magnetic fields in different directions to distinguish NQR from acoustic ringing induced in the sample.
2. Description of the Related Art
There are many situations where it is desirable to detect the presence of a target material (that is, a specific substance). For example, with the unfortunate increase in drug trafficking and terrorist use of high explosives in aircraft and buildings, detection systems are often used to detect sub-kilogram quantities of narcotics and explosives against a background of more benign materials. For example, such detection systems are typically used in airports to detect narcotics or explosives hidden in luggage.
Nuclear quadrupole resonance (NQR) is a known technique for detecting a target material. Generally, radio frequency (RF) radiation at a particular frequency will induce a free induction decay (FID) from nuclear spins in specific substances, but not in other substances. Nuclear quadrupole resonance (NQR) takes advantage of this phenomenon to detect one of these specific substances as a target material.
FIG. 1 is a diagram illustrating a conventional NQR apparatus. Referring now to FIG. 1, a transmitter 20 and a receiver 22 are connected to a probe 24 through a transmit/receive (T/R) switch 26. Probe 24 includes an inductor, such as a coil 28, forming part a resonant circuit with various other inductors L and capacitors C. To detect the presence of a target material, T/R switch 26 connects transmitter 20 to probe 24 while disconnecting receiver 22 from probe 24. Then, transmitter 20 generates a pulse and supplies the pulse to probe 24. Generally, the pulse is formed by an RF signal having a frequency corresponding to the resonance signal of the nuclei of the target material which is intended to be detected. Probe 24 receives the pulse, which causes coil 28 to store (RF) energy. If a sample (not illustrated) is appropriately placed near, or inside, coil 28, the stored RF energy will cause a corresponding RF magnetic field to irradiate the sample. If the sample includes the target material, the RF magnetic field will induce a nuclear quadrupole resonance (NQR) in the target material. The NQR signal is the free induction decay (FID) from nuclear spins in the target material that were excited by the RF magnetic field.
After the sample is irradiated with the RF magnetic field, T/R switch 26 connects receiver 22 to probe 24 while disconnecting transmitter 20 from probe 24. Coil 28 then detects the NQR induced in the target material, and probe 24 produces a corresponding output signal. The output signal of probe 24 is received and analyzed by receiver 22, to confirm the presence of the target material in the sample.
In real world use of NQR to detect narcotics and explosives, a sample may contain nearly any substance known to man, and may possibly include the explosive or narcotic to be detected. Because of the narrow bandwidth of an appropriate RF magnetic field irradiating the sample and the large range of NQR frequencies in benign materials, it is unlikely that an NQR signal will be induced in the other substances by the RF magnetic field. As a result, an NQR apparatus can accurately detect specific substances without producing false alarms (false positives).
However, the RF magnetic field can induce magnetostrictive ringing in a sample containing magnetic domains, due to a magnetostrictive effect. Similarly, the electric field component of the RF magnetic field can induce piezoelectric ringing in the sample. Magnetostrictive ringing and piezoelectric ringing can both be referred to as xe2x80x9cacoustic ringingxe2x80x9d. Unfortunately, such acoustic ringing can be relatively large, and can produce false alarms in the detection of a target material.
The number of false alarms can be reduced by recognizing that acoustic ringing and NQR respond differently to an RF magnetic field. Therefore, acoustic ringing and NQR can be distinguished in many cases. However, the character of acoustic ringing often changes over time, requiring that acoustic ringing cancellation schemes be implemented on a short time scale. In addition, due to the time variation of acoustic ringing and the sometimes very large difference in amplitudes between acoustic ringing and NQR, it can be difficult to detect NQR.
Accordingly, it is an object of the present invention to provide an NQR apparatus and method which provide accurate detection of NQR, while reducing the likelihood of false alarms.
It is a further object of the present invention to provide an NQR apparatus and method which can separate NQR from acoustic ringing induced in a sample.
Additional objects and advantage of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may learned by practice of the invention.
The foregoing objects of the present invention are achieved by providing a method which includes the steps of (a) generating an RF magnetic field in a first direction towards a sample; and (b) generating an RF magnetic field in a second direction towards the sample, wherein the second direction is not parallel to the first direction, and the RF magnetic fields are generated to detect NQR in the sample.
Objects of the present invention are further achieved by providing a method which includes the steps of (a) generating an RF magnetic field along an RF magnetic field axis towards a sample to cause a resonance signal in the sample, the resonance signal including NQR and acoustic ringing; (b) detecting the resonance signal along the RF magnetic field axis; and (c) detecting the acoustic ringing along a direction not parallel to the RF magnetic field axis, so that the NQR in the resonance signal can be distinguished from the acoustic ringing in the resonance signal.
Objects of the present invention are achieved by providing a probe which generates an RF magnetic field in a first direction and an RF magnetic field in a second direction towards a sample, to detect NQR in the sample. The first and second directions are not, parallel to each other, and are preferably orthogonal.
Objects of the present invention are also achieved by providing a probe which generates an RF magnetic field along an RF magnetic field axis towards a sample to cause a resonance signal in the sample. The resonance signal includes NQR and acoustic ringing. The probe detects the resonance signal along the RF magnetic field axis and detects the acoustic ringing along a direction not parallel to the RF magnetic field axis, so that the NQR in the resonance signal can be distinguished from the acoustic ringing in the resonance signal.